Interior permanent magnet (IPM) machines are favored for fuel cell and hybrid electric vehicle operations due to their desirable characteristics—i.e., good torque density, good overall efficiency, good constant power range, etc. The rotor field in a permanent magnet machine is obtained by virtue of its structure, unlike other machines such as induction, switched or synchronous reluctance machines, in which the field is generated by a stator current supplied by a source. As a result, permanent magnet machines exhibit superior efficiency as compared to other such machines.
However, as with surface PM machines, an IPM machine is burdened by the fact that the permanent magnet field is present even when the machine is not powered, resulting in losses induced by the rotating permanent magnet field of the rotor. Furthermore, the permanent magnet field induces voltage (“back EMF”) into the stator winding. For a strong permanent magnet machine, this back EMF can be quite significant.
In an IPM machine, a second rotor barrier is sometimes added, and a small magnet (used to saturate the bridge above the rotor barrier) is optionally placed therein. This second layer of magnet or air pocket acts as a barrier to the permanent magnet field of the lower primary magnet layer, reducing the air-gap magnet flux, and also lowering the machine back EMF and losses induced by the permanent magnet field. For some machines, due to limited space, the second barrier can not easily be added. Furthermore, addition of the second barrier may weaken the rotor structure or may not cover the entire lower magnet, resulting in some portion of the lower barrier magnet directly exposed to the air-gap, in turn leading to higher losses and higher back EMF.
Accordingly, it is desirable to provide IPM machines that reduce field-related losses while minimizing back EMF and air-gap magnetic flux. Other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.